It has been observed that devices such as set top boxes, cable modems, and embedded multimedia terminal adapters (“eMTA”s) can suffer from performance degradation when radiated interferers such as cordless telephones, cellular telephones and wireless routers are placed in proximity to them. The performance degradation can take the form of increased bit error rate, or in the extreme case, complete loss of the input signal. With a set top box, the performance degradation can result in degraded picture quality or complete loss of picture. With a cable modem, performance degradation will result in lost data. With an eMTA, increased bit error rate can result in poor voice quality or complete loss of calls.
When products with this susceptibility are deployed, users are advised to increase the distance between the susceptible device and the interfering device. This is referred to as the Safe Operating Area. As this is operationally an undesirable solution to the problem, design techniques are required to solve the problem at the point of the susceptibility, namely the RF tuner of the settop, cable modem or eMTA.
In more detail, the problem is manifested when the local oscillator (“LO”) of a broadband tuner has harmonics that fall within the bandwidth of a radiating interferer. U.S. Pat. No. 6,057,876, for a “Dual conversion set-top terminal including a system for avoiding oscillator difference beats,” discloses the manipulation of 1st and 2nd LO's and 1st intermediate frequency (“IF”) bandwidth to improve performance of a tuner relative to oscillator difference beats. However, amongst other things the disclosure does not address the problem of radiating interferers.
Another attempted solution for this problem is to use heavy mechanical shielding of the tuner subsystem to reduce the magnitude of the problem. However, even such mechanical efforts at reducing degradation are limited.